消失的密码学：PDF技术与信息安全的碰撞

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"Disappearing Cryptography.pdf" "Disappearing Cryptography"这本书深入探讨了在信息安全领域中的一个独特概念——自消失加密技术。该技术涉及的是加密数据在一定时间或条件下会自动消失或变得不可读，以此来提高数据的安全性和隐私保护。作者Peter Wayner在书中详细阐述了这种技术的工作原理、设计思路以及可能的应用场景。 Delphi，作为一个标签，可能是指书中提到的一种编程语言或开发工具。Delphi是Embarcadero Technologies开发的一款集成开发环境（IDE），它基于Object Pascal语言，广泛用于创建Windows和Web应用程序。在与自消失加密相关的上下文中，Delphi可能会被用来编写实现这类加密算法的软件程序。书中内容提到了出版商Morgan Kaufmann Publishers以及版权信息，强调了对知识产权的尊重。作者提醒读者，虽然产品名称在书中通常以首字母大写或全大写的形式出现，但这并不意味着这些名称没有被注册为商标。如果读者想要获取关于商标和注册的更多信息，应直接联系相关公司。同时，书籍的复制和传播受到严格的版权保护，未经许可，不得以任何形式复制、存储或传输。此外，书中的内容还提到了获取版权许可的途径，包括直接联系Elsevier的科学与技术权益部，或者通过Elsevier的官方网站在线申请。这反映了出版业对于版权管理的规范流程，确保了作者的权益不受侵犯。自消失加密技术在现代网络安全中具有重要价值，特别是在短期通信、临时数据交换以及合规性要求较高的行业中，如金融、医疗和政府。这种技术结合了密码学和时间敏感的销毁机制，为数据生命周期管理提供了一种创新方法，既能满足信息即时共享的需求，又能防止数据长期留存带来的潜在风险。在实际应用中，自消失加密可以应用于瞬时消息服务、安全文件分享平台以及各种需要保护隐私的数据交互场合。 "Disappearing Cryptography"这本书为读者提供了一个全面理解自消失加密技术的窗口，同时也展示了如何利用如Delphi这样的工具来实现此类技术。它不仅是一本关于密码学的学术著作，也是信息安全实践者和软件开发者的重要参考资源。

CHAPTER 1. FRAMING INFORMATION

Many of these spreading techniques hide information in the

noise of an image or sound ﬁle, but there is no reason why they

can’t be used with other forms of data as well.

Many of the techniques

are closely related to the

process of generating

cryptographically secure

random numbers– that

is, a stream of random

numbers that can’t be

predicted. Some

algorithms use this

number stream to

choose locations, others

blend the random

values with the hidden

information, still others

replace some of the

random values with the

message.

Adopt a Statistical Proﬁle Data often falls into a pattern and com-

puters often try to make decisions about data by looking at t he

pattern. English text, for instance, uses the letter ‘p’ for more

often t han the letter ‘q’ and this information can b e useful for

breaking ciphers. If data can be reformulated so it adopts the

statistical proﬁle of the English language, then a computer pro-

gram minding ps and qs will be fooled.

Adopt a Structural Proﬁle Mimicking the statistics of a ﬁle is just

the beginning. More sophisticated solutions rely on complex

models of the underlying data to better mimic it. Chapter 7, for

instance, hides information by making it look like the transcript

of a baseball game. The bits are hidden by using them to choose

between the nouns, verbs and other parts of the text. T he data

are recovered by sorting through the text and matching up the

words with the bits that selected them. This technique can

produce startling results, although the content of the messages

often seems a bit loopy or directionless. This is often good

enough to fool humans or computers that are programmed to

algorithmically scan for particular words or patterns.

Replace Randomness Many software programs use random num-

ber generators to add realism to scenes, sounds, and games.

Monsters look better if a random number generator adds blotches,

warts, moles, scars and gouges to a smooth skin deﬁned by

mathematical spheres. Information can be hidden in the place

of the random number. The location of the splotches and scars

carries the message.

Change the Order A grocery list may be just a list, but the order of

the items can carry a surprisingly large amount of information.

Split Information Data can be split into any number of packets that

take different routes to their destination. Sophisticated algo-

rithms can also split the information so that any subset of k of

the n parts are enough to reconstruct the entire message.

Hide the Source Some algorithms allow people to broadcast infor-

mation without revealing their identity. This is not t he same as

hiding the information itself, but it is still a valuable tool. Chap-

ters 10 and 11 show how to use anonymous remailers and more

mathematically sophisticated Dining Cryptographers’ solutions

to distribute information anonymously.

These different techniques can be combined in many ways. First

information can be hidden by hiding it in a list, then the list can be

hidden in the noise of a ﬁle that is then broadcast in a way to hide the

source of the data.

1.0.3 How Steganography Is Used

Hidden information has a variety of uses in products and protocols.

Hiding slightly different information or combining the various algo-

rithms creates different tools with different uses. Here are some of

the most interesting applications:

Enhanced Data Structures Most programmers know that standard

data structures get old over time. Eventually there comes a time

when new, unplanned information must be added to the for-

mat without breaking old software. Steganography is one so-

lution. You can hide extra information about the photos in the

photos themselves. This information travels with the photo but

will not disturb old software that doesn’t know of its existence.

A radiologist could embed comments from in the background

of a digitized x-ray. The ﬁle would still work with standard tools,

saving hospitals the cost of replacing all of their equipment.

Strong Watermarks The creators of digital content like books, movies,

and audio ﬁles want to add hidden information into the ﬁle

to describe the restrictions they place on the ﬁle. This mes-

or as complex as “This ﬁle can only be played twice before

12/31/2002 unless you purchase three cases of soda and sub-

mit their bottle tops for rebate. In which case you get 4 song

plays for ever y bottle top.”

Digital Watermarking

by Ingemar J. Cox,

Matthew L. Miller and

Jeffrey A. Bloom is a

good introduction to

watermarks and the

challenges particular to

the subﬁeld.[CMB01]

Some watermarks are meant to be found even after the ﬁle un-

dergoes a great deal of distortion. Ideally, the watermark will

still be d etectable even after someone crops, rotates, scales and

compresses some document. The only way to truly destroy it is

to alter the document so much that it is no longer recognizable.

Other watermarks are deliberately made as fragile as possible.

If someone tries to tamper with the ﬁle, the watermark will

disappear. Combining strong and weak watermarks is a good

option when tampering is possible.

CHAPTER 1. FRAMING INFORMATION

Document-Tracking Tools Hidden information can identify the le-

gitimate owner of the document. If it is leaked or distributed

to unauthorized people, it can be tracked back to the rightful

owner. Adding individual tags to each document is an idea at-

tractive to both content-generating industries and government

agencies with classiﬁed information.

File Authentication The hidden information bundled with a ﬁle can

also contain a digital signature certifying its authenticity. A reg-

ular software program would simply display (or play) the doc-

ument. If someone wanted some assurance, the digital signa-

ture embedded in the document can verify that the right person

signed it.

Private Communications Steganography is also useful in political

situations when communications is dangerous. There will al-

ways be moments when two people can’t exchange messages

because their enemies are listening. Many governments con-

tinue to see the Internet, corporations and electronic conver-

sations as an opportunity for surveillance. In these situations,

hidden channels offer the politically weak a chance to elude the

powerful who control the networks. [Sha01]

Not all uses for hidden information come classiﬁed as stegano-

graphy or cryptography. Anyone who deals with old data formats and

old software knows that programmers don’t always provide ideal data

structures with full documentation. Many basic hacks aren’t much

differe nt from the steganographic tools in this book. Clever program-

mers ﬁnd additional ways to stretch a data format by packing extra

information where it wasn’t needed before. This kind of hacking is

bound to yield more applications than people imagined for stegano-

graphy. Somewhere out there, a child’s life may be saved thanks to

clever data handling and steganography!

1.0.4 Attacks on Steganography

Steganographic algorithms provide stealth, camouﬂage and security

to information. How much, though, is hard to measure. As data

blends into the background, when does it effectively disappear? One

way to judge the strength is to imagine different attacks and then

try to determine whether the algorithm can successfully withstand

them. This approach is far from perfect, but it is the best available.

There’s no way to anticipate all possible attacks, although you can try.

Attacking steganographic algorithms is very similar to attack-

ing cryptographic algorithms and many of the same techniques ap-

ply. Of course, steganographic algorithms promise some additional

stealth in addition to security so they are also vulnerable to addi-

tional attacks.

Here’s a list of some possible attacks:

File Only The attacker has access to the ﬁle and must determine if it

holds a hidden message. This is the weakest form of attack, but

it is also the minimum threshold for successful steganography.

Many of these basic attacks rely on a statistical analysis of digi-

tal images or sound ﬁles to reveal the presence of a message in

the ﬁle. This type of attack is often more of an art than a sci-

ence because the person hiding the message can try to counter

an attack b y adjusting the statistics.

File and Original Copy In some cases, the attacker may have a copy

of the ﬁle with the encoded message and a copy of the original,

pre-encoded ﬁle. Clearly, detecting some hidden message is a

trivial operation. If the two ﬁles are different, there must be

some new information hidden inside of it.

The real question is what the attacker may try to do with the

data. The attacker may try to destroy t he hidden information,

something that can be accomplished by replacing it with the

original. The attacker may try to extract the information or

even replace it with their own. The best algorithms try to de-

fend against someone trying to forge hidden information in a

way that it looks like it was created by someone else. This is of-

ten imagined in the world of watermarks, wher e the hidden in-

formation might identify the rightful owner. An attacker might

try to remove the watermark from a legitimate owner and re-

place it with a watermark giving themselves all of the rights and

privileges associated with ownership.

Multiple Encoded Files The attacker gets n different copies of the

ﬁles with n different messages. One of them may or may not

be the original unchanged ﬁle. This situation may occur if a

company is inserting different tracking information into each

ﬁle and the attacker is able to gather a number of different ver-

sions. If music companies sell digital sound ﬁles with person-

alized watermarks, then several fans with legitimate copies can

get together and compare their ﬁles.

Some attackers may try to destroy the tracking information or

to replace it with their own version of the information. One of

CHAPTER 1. FRAMING INFORMATION

the simplest attacks in this case is to blend the ﬁles t ogether,

either by averaging the individual elements of the ﬁle or by

creating a hybrid by taking different parts from each ﬁle.

Access to the File and Algorithm An ideal steganographic algorithm

can withstand scrutiny even if the attacker knows the algorithm

itself. Clearly, basic algorithms that hide and unveil informa-

tion can’t resist this attack. Anyone who knows the algorithm

can use this it to extract the information.

But this can work if you keep some part of the algorithm se-

cret and use it as the “key” to unlock the information. Many

algorithms in this book use a cryptographically secure random

number generator to control how the information is blended

into a ﬁle. The seed value to this random number stream acts

like a key. If you don’t know it, you can’t generate the random

number stream and you can’t unblend the information.

Destroy Everything Attack Some people argue that steganography

is not particularly useful because an attacker could simply de-

stroy the message by blurring a photo or adding noise to a

sound ﬁle. One common technique used against the kind of

block compression algorithms like JPEG is to rotate an image

45 degrees, blur the image, sharpen it again, and then rotate it

back. This mixes information from different blocks of the im-

age, effectively removing some schemes like the ones in Chap-

ter 14.

This technique is a problem, but it can be computationally pro-

hibitive for many users and it introduces its own side effects.

A site like Flickr.com might consider doing this to all incom-

ing images to deter communications, but it would requir e a fair

amount of computation.

It is also not an artful attack. Anyone can destroy messages.

Cryptography and many other protocols are also vulnerable to

it.

Random Tweaking Attacks Some attackers may not try to deter-

mine the existence of a message with any certainty. An attacker

could just add small, random tweaks to all ﬁles in the hope of

destroying whatever message may be there. During World War

II, the government censors would add small changes to num-

bers in telegrams in the hopes of destroying covert communi-

cations. This approach is not very useful because it sacriﬁces

overall accuracy for the hope of squelching a message. Many

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